As one of the fiber laser devices used in, for example, a processing machine and medical equipment, there has been known an MO-PA (Master Oscillator-Power Amplifier) fiber laser device which amplifies light, generated by a seed light source such as a laser oscillator (MO: Master Oscillator), by an amplifier (PA: Power Amplifier) and outputs the light. As an amplifier used in the amplifier, there has been known an optical fiber amplifier which amplifies light by an amplification optical fiber.
The amplification optical fiber generally uses a double-clad fiber in which an active element such as a rare-earth element is added to the core. The double-clad fiber includes a double-clad fiber whose core propagates only single mode light and a double-clad fiber whose core propagates multi-mode light. In the double-clad fiber whose core propagates only single mode light, the cross-sectional area of the core is small, and therefore, if a high power laser output is to be obtained, the density of light propagating through the core may be excessively increased. In this case, the energy of light is transferred to an undesired wavelength by a nonlinear optical effect, and an expected laser output may not be obtained. Thus, with the recent requirements for realization of high output of an optical fiber amplifier, an optical fiber amplifier using the double-clad fiber whose core propagates multi-mode light has attracted attention.
In the optical fiber amplifier, it is preferable in terms of improving the beam quality of output light that an LP01 mode of propagating light be amplified, and other higher-order modes not be amplified. In the following patent documents, examples of such an optical fiber amplifier are described. In the optical fiber amplifier, a mode converter which excites only the LP01 mode of light is provided, whereby amplification can be performed around the LP01 mode even in an amplification double-clad fiber which propagates multi-mode light. Patent Document 1 further suggests that by using an amplification optical fiber in which an active element is added to the central portion of the core of the double-clad fiber, and the active element is not added to an outer peripheral portion of the core, the LP01 mode is amplified more effectively than higher-order modes by the effect of a gain guided wave.
Patent Document 2 describes the following idea. That is, by using an amplification optical fiber in which an active element is added to the central portion of the core of the double-clad fiber, and an absorption element absorbing light is added to an outer peripheral portion of the core, unnecessary higher-order modes are attenuated.    [Patent Document 1] U.S. Pat. No. 5,818,630    [Patent Document 2] U.S. Pat. No. 5,121,460